doc: various improvements

- g_subprocess_launcher_spawn() and spawnv(): there is no other way
  AFAIK to create a GSubprocess from a launcher. So these
  functions are not "convenience helper".

- annotate optional arguments for g_shell_parse_argv().

- other trivial fix

https://bugzilla.gnome.org/show_bug.cgi?id=732357

gio/gsubprocess.c

@@ -1769,7 +1769,7 @@ g_subprocess_communicate_utf8 (GSubprocess   *subprocess,
  * @callback: Callback
  * @user_data: User data
  *
- * Asynchronous version of g_subprocess_communicate_utf().  Complete
+ * Asynchronous version of g_subprocess_communicate_utf8().  Complete
  * invocation with g_subprocess_communicate_utf8_finish().
  */
 void

gio/gsubprocesslauncher.c

@@ -675,8 +675,7 @@ g_subprocess_launcher_set_child_setup (GSubprocessLauncher  *self,
  * @argv0: Command line arguments
  * @...: Continued arguments, %NULL terminated
  *
- * A convenience helper for creating a #GSubprocess given a provided
- * varargs list of arguments.
+ * Creates a #GSubprocess given a provided varargs list of arguments.
  *
  * Since: 2.40
  * Returns: (transfer full): A new #GSubprocess, or %NULL on error (and @error will be set)
@@ -719,8 +718,7 @@ g_subprocess_launcher_spawn (GSubprocessLauncher  *launcher,
  * @argv: (array zero-terminated=1) (element-type utf8): Command line arguments
  * @error: Error
  *
- * A convenience helper for creating a #GSubprocess given a provided
- * array of arguments.
+ * Creates a #GSubprocess given a provided array of arguments.
  *
  * Since: 2.40
  * Returns: (transfer full): A new #GSubprocess, or %NULL on error (and @error will be set)

glib/gshell.c

@@ -619,9 +619,10 @@ tokenize_command_line (const gchar *command_line,
 /**
  * g_shell_parse_argv:
  * @command_line: command line to parse
- * @argcp: (out): return location for number of args
- * @argvp: (out) (array length=argcp zero-terminated=1): return location for array of args
- * @error: return location for error
+ * @argcp: (out) (optional): return location for number of args, or %NULL
+ * @argvp: (out) (optional) (array length=argcp zero-terminated=1): return
+ *   location for array of args, or %NULL
+ * @error: (optional): return location for error, or %NULL
  * 
  * Parses a command line into an argument vector, in much the same way
  * the shell would, but without many of the expansions the shell would